The present invention relates to an epitaxial growth structure for a semiconductor light-emitting device using a sapphire substrate, and a semiconductor light-emitting device using the same.
As a light-emitting device material in a region of wavelengths (wavelength=480 nm or less) shorter than that of blue, various types of light-emitting devices using ZnSe.sub.x S.sub.1-x (0.ltoreq.x.ltoreq.1), SiC, and GaN have been proposed, and their samples have been manufactured.
A sapphire (0001) face (C face) or a (2110) face (A face) is used for GaN.
In order to realize a high-efficiency light-emitting device, the material is ideally a direct transition semiconductor material, enables current injection by a p-n junction, and can manufacture a double hetero (DH) structure in which a light-emitting layer and p- and n-type cladding layers are lattice-matched. Problems of the above light-emitting device will be described below.
(a) ZnSe.sub.x S.sub.1-x (0.ltoreq.x.ltoreq.1)
Film formation is attempted on a GaAs substrate by an MOVPE (Metalorganic Vapor Phase Epitaxy) method, an MBE (Molecular Beam Epitaxy) method, and the like. In these methods, by changing a composition ratio of Se to S, a lattice constant and a band gap energy can be changed. In this material system, however, the band gap energy is solely determined if the lattice constant is determined. Therefore, no DH structure cannot be formed under lattice matching conditions.
(b) SiC
A p-n junction can be formed by doping a suitable impurity. This material, however, is an indirect transition semiconductor. In addition, no DH structure can be formed since no materials having the same lattice constant and different band gap energies are available.
(c) GaN on Sapphire C- or A-face Substrate
Since N holes serving as a donor are present at a high concentration, no p-type material is realized. That is, all of announced light-emitting devices have a structure (MIS type) in which a metal electrode is formed on an n-type GaN thin film via a GaN layer semi-insulated by Zn or Mg doping. This light-emitting device utilizes a phenomenon in which, after a high electric field is applied to the Zn- or Mg-doped layer to ionize a light emission center, light is emitted when electrons drop to their original energy level. A light-emitting device using GaN grown directly on the substrate has a low quantum efficiency. This GaN crystal has the following problems.
.circle.1 Significant three-dimensional growth is caused (a three-dimensional pattern corresponding to a film thickness).
.fwdarw.Severe problems in manufacturing a light-emitting device having a multilayered structure are posed, i.e., an electric field cannot be uniformly applied since the film thickness is not uniform, and carrier injection cannot be performed, thereby largely degrading the device characteristics. PA1 .fwdarw.It becomes difficult to perform conductivity control, and a light absorption coefficient in the material is significantly increased. PA1 .fwdarw.An improvement in crystallinity is limited, resulting in a low quantum efficiency, a short living, and a low optical output of a light-emitting device.
.circle.2 N hole (serving as a donor) concentration is high (carrier concentration=10.sup.18 to 10.sup.19 cm.sup.-3)
.circle.3 Large lattice mismatching, or noncoincidence of crystallographic symmetries between a substrate and an epitaxial growth film (Table 1; Note that the lattice mismatching is obtained by comparing minimum period units of crystals).
TABLE 1 __________________________________________________________________________ Lattice Substrate-Epitaxial Misalign- Film Interface Orientation Relationship ment Symmetry __________________________________________________________________________ (0113)GaN/Sapphire M Face [03- 3- 2]GaN//[2- 1- 10]Sapphire -2.6% Coincidence [2- 1- 10]GaN//[0001]Sapphire 1.9% (0001)GaN/Sapphire C Face [2- 1- 10]GaN//[01- 10]Sapphire 13.8% Coincidence [01- 10]GaN//[- 2110]Sapphire 13.8% (0001)GaN/Sapphire A Face [01- 10]GaN//[01- 10]Sapphire -0.4% Noncoincidence [- 2110]GaN//[0001]Sapphire 1.9% (2- 1- 10)GaN/Sapphire R Face [01- 10]GaN//[- 2110]Sapphire 13.8% Coincidence [0001]GaN//[0- 111]Sapphire 1.1% __________________________________________________________________________
Note that Table 1 compares lattice mismatching and crystallographic symmetry between a sapphire substrate and a GaN film with respect to a substrate orientation.
The above items .circle.1 and .circle.2 are caused by the following reasons.
.circle.1 In growth on the C face, the number of nuclear formation sites is small, and coarsely formed growth nuclei individually form hillocks.
.circle.2 The GaN film on the C- and A-face substrate is (0001)-axis oriented to be a Ga termination. As a result, an adhesion coefficient of N is small and an N hole concentration is high on the Ga termination face. This state is schematically shown in FIG. 1A.
Referring to FIGS. 1A and 1B, solid circles represent Ga, and open circles represent N. The symbols " .circle." and " .circle." represent atoms on the drawing surface, and " .circle." and " .circle." represent atoms in front of the drawing surface. In addition "--" indicates that only parallel bonds are present on the drawing surface, and ".dbd." indicates that an atom on the drawing surface is bonded to atoms before and behind the drawing surface.
In order to solve the problems .circle.1 and .circle.2 , a method in which AlN is used as a buffer layer between sapphire and GaN is reported. AlN serves as a buffer layer to reduce an interface energy between sapphire and GaN to accelerate dense nuclear formation. As a result, the problem .circle.1 is almost solved, and the problem .circle.2 is partially solved. Since, however, the problems .circle.2 and .circle.3 are essential problems in this system, the method using the AlN buffer layer is unsatisfactory in a practical device.
As described above, none of blue light-emitting devices currently proposed is practical mainly because the crystallinity of an active layer is insufficient.